Investors have paid $55M for a minority stake in a River North office tower, two months after it was purchased for $138M by Boston-based Beacon Capital Partners. Callahan Capital and Montreal partner Ivanhoe Cambridge purchased the 40% stake in the 683K SF office building, which was until recently the HQ of the American Medical Association. The office building is about 57% vacant thanks to the departure of the AMA, which took up over half the building, Crain's reports. That's compared to the area's overall office vacancy rate of just 13.3%, according to CBRE. Callahan Capital and Ivanhoe Cambridge leaders say the significant available space in the popular neighborhood is an asset. They are planning to renovate the tower's lobby and some floors. In 2013 the investors paid $361M for two office buildings in the West Loop; this venture brings them to a total of 2.1M SF of office space in Chicago. [Crain’s]